As portable wireless devices such as a video camera, a portable phone, a portable computer, and the like, reduce weight and are highly functionalized, researches on secondary batteries used as a driving power source thereof have been conducted a lot. The secondary batteries include, for example, a nickel cadmium battery, a nickel hydrogen battery, a nickel zinc battery, a lithium secondary battery and the like. Among the batteries, the lithium secondary battery is rechargeable, has a small size and a large capacity, and has a high operating voltage and a high energy density per unit weight. Thus, the lithium secondary battery is widely used in high-tech electronic device fields.
Lithium generally used as a material for a secondary battery has a small atomic weight and is an appropriate material for manufacturing a battery having a large electric capacity per unit weight. Meanwhile, since lithium vigorously reacts with water, a non-aqueous electrolyte is used in a lithium-based battery. In this case, without being affected by an electrolysis voltage of water, an electromotive force of about 3 V to 4V may be generated in the lithium-based battery.
Lithium secondary batteries mainly use a lithium-based oxide as a positive electrode active material and a carbon-based material as a negative active material. Generally, the lithium secondary batteries are classified into a liquid electrolyte battery and a polymer electrolyte battery according to the kind of the electrolyte. The battery using the liquid electrolyte is called as a lithium ion battery and the battery using the polymer electrolyte is called as a lithium polymer battery. In addition, the lithium secondary battery is manufactured in various shapes, and typically includes a cylinder shape, a polygon shape and a pouch shape.
Commonly, the lithium secondary battery includes an electrode assembly including the winding structure of a positive electrode coated with a positive electrode active material, a negative electrode coated with a negative electrode active material and a separator disposed between the positive electrode and the negative electrode for preventing short and permitting the migration of lithium ions, a case for receiving the electrode assembly and an electrolyte injected into the case so as to permit the migration of the lithium ions.
In the lithium secondary battery, the short between the electrodes may be easily generated during charging/discharging, and the increase of pressure or the deteriorating phenomenon of the electrode according to overcharging and over-discharging may be easily generated. These are factors threatening the safety of the lithium secondary battery.
When the lithium secondary battery is overcharged, the electrolyte may be vaporized from about the upper portion of the electrode assembly to increase the resistance of the battery. In addition, the transformation of the electrode assembly may begin from about the center portion thereof to precipitate lithium. Of course, a local heating may begin according to the increase of the resistance at the upper portion of the electrode assembly, and the temperature of the battery may be rapidly increased. In this state, the internal pressure may be rapidly increased by an electrolyte additive such as cyclohexylbenzene (CHB) and biphenyl (BP), which may be generally decomposed and generate a gas during the overcharging. When the amount of the electrolyte additive such as cyclohexylbenzene (CHB) and biphenyl (BP) is increased, the amount of the gas generated during the overcharging may be increased. However, in this case, the capacity or the quality of the battery may be deteriorated and the lifetime of the battery may be decreased.
In order to solve the above-described defects, a material possibly being electrolyzed at a certain voltage during overcharging and generating a gas may be included in the electrode active material to increase the safety of the battery and to make the battery slim when compared with the battery including the electrolyte additive. However, when the material for rapidly exhausting a large amount of a gas during overcharging is added in the positive electrode active material, the amount of the positive electrode active material may be relatively decreased. In this case, the battery capacity may be decreased, the resistance may be increase, and the lifetime of the battery may be decreased due to repeated charging/discharging.
Accordingly, researches on a lithium secondary battery preventing the danger of explosion during overcharging, having an improved safety of a positive electrode active material during charging in order to satisfy a high capacity and a slim structure, operating a safety device such as a safety vent and a PTC device at a proper time, and having a minimized capacity deterioration of the battery, are required.